NatTypeTester/STUN/Message/Attribute.cs

using STUN.Enums;
using STUN.Message.Attributes;
using STUN.Utils;
using System;
using System.Collections.Generic;
using System.Linq;

namespace STUN.Message
{
	/// <summary>
	/// https://tools.ietf.org/html/rfc5389#section-15
	/// </summary>
	public class Attribute
	{
		/*
            Length 是大端
            必须4字节对齐
            对齐的字节可以是任意值
             0                   1                   2                   3
             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |         Type                  |            Length             |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |                         Value (variable)                ....
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         */

		public AttributeType Type { get; set; } = AttributeType.Useless;

		public ushort Length { get; set; }

		public int RealLength => Type == AttributeType.Useless ? 0 : 4 + Length + (4 - Length % 4) % 4;

		public IAttribute? Value { get; set; }

		private readonly byte[] _magicCookie;
		private readonly byte[] _transactionId;

		public Attribute()
		{
			_magicCookie = new byte[4];
			_transactionId = new byte[12];
		}

		public Attribute(byte[] magicCookie, byte[] transactionId)
		{
			if (magicCookie.Length != 4 || transactionId.Length != 12)
			{
				throw new ArgumentException(@"Wrong Transaction ID length");
			}

			_magicCookie = magicCookie;

			_transactionId = transactionId;
		}

		public IEnumerable<byte> ToBytes()
		{
			var res = new List<byte>();

			res.AddRange(Convert.ToUInt16(Type).ToBe());
			res.AddRange(Length.ToBe());
			if (Value != null)
			{
				res.AddRange(Value.Bytes);
			}

			var n = (4 - res.Count % 4) % 4; // 填充的字节数
			res.AddRange(BitUtils.GetRandomBytes(n));

			return res;
		}

		/// <returns>
		/// Parse 成功字节，0 则表示 Parse 失败
		/// </returns>
		public int TryParse(byte[] bytes)
		{
			if (bytes.Length < 4)
			{
				return 0;
			}

			Type = (AttributeType)BitUtils.FromBe(bytes[0], bytes[1]);

			Length = BitUtils.FromBe(bytes[2], bytes[3]);

			if (bytes.Length < 4 + Length)
			{
				return 0;
			}

			var value = bytes.Skip(4).Take(Length).ToArray();

			IAttribute t = Type switch
			{
				AttributeType.MappedAddress => new MappedAddressAttribute(),
				AttributeType.XorMappedAddress => new XorMappedAddressAttribute(_magicCookie, _transactionId),
				AttributeType.ResponseAddress => new ResponseAddressAttribute(),
				AttributeType.ChangeRequest => new ChangeRequestAttribute(),
				AttributeType.SourceAddress => new SourceAddressAttribute(),
				AttributeType.ChangedAddress => new ChangedAddressAttribute(),
				AttributeType.OtherAddress => new OtherAddressAttribute(),
				AttributeType.ReflectedFrom => new ReflectedFromAttribute(),
				AttributeType.ErrorCode => new ErrorCodeAttribute(),
				_ => new UselessAttribute()
			};
			Value = t.TryParse(value) ? t : null;

			return 4 + Length + (4 - Length % 4) % 4; // 对齐
		}
	}
}